1. Field of the Invention
The present invention relates to a track seeking control apparatus in use with a data record/reproduction system which has at least one of the functions to record and reproduce data to and from a recording medium with tracks, and to erase data as stored in the recording medium.
2. Description of the Related Art
Recently, data record/reproduction systems of the type in which data is recorded in high density into a recording medium by a laser beam, and is reproduced from the same, have come in. A typical example of those systems is an optical disk system. An interval between the adjacent tracks on an optical disk is usually 1.6 .mu.m. This figure indicates that the optical disk is capable of recording data in much higher recording density than other types of memories. The optical disk system, however, requires an extremely fine control technique to exactly position an optical head at a desired track on the optical disk, when compared with another type of disk memory.
A known seeking control system for positioning the optical head at a desired or target track on the optical disk, counts track cross signals generated when the head crosses the tracks, and seeks a target track on the basis of the count. Another known seeking control system moves the optical head toward a target track with reference to an external scale. The former may more exactly position the head than the latter. The tracks on the optical disk are generally divided into a plurality of data areas (referred to as sectors). The head of each sector contains a portion where a track or a groove terminates. This portion of the sector is used for prestoring a track number or a sector number, or as a mirror portion (or a mirror mark) to remove a track offset, which is essential to the push-pull tracking system. To seek a target track on the optical disk thus arranged, the optical head is moved while counting track cross signals. When a light spot emitted by the head passes the groove portion during the seek operation, the head may fail to detect and count a track cross signal. Therefore, the counting of the track cross signal is incorrect.
To solve such a problem, Japanese Patent Application Unexamined Publication No. 59-221878 proposed the seeking control system. A circuit arrangement implementing the seeking control system is shown in FIG. 1. A track cross pulse as read by an optical head (not shown) is detected through a combination of a NOT circuit 1, flip-flops 2 and 3, and an AND gate 4. The cross pulse signal is applied through an OR gate 5 to a cross pulse counter 6, and is counted by the counter 6. An inter-pulse counter 8 counts a value of an interval between the adjacent cross pulses with reference to reference pulses generated by a reference pulse generator 7. The pulse interval value is applied through an AND gate 9 to a count memory 11. A divider 10 divides the pulse interval value as inputted from the inter-pulse counter 8 by the previous pulse interval value as stored in the count memory 11. If the track cross pulses are normally counted, the pulse interval values are equal and hence an output signal of the divider 10 represents 1. Under this condition, an output signal of a -1 circuit 12, which subtracts 1 from the output signal of the divider 10, represents 0. Therefore, a pulse generator 13 produces no pulse. When one pulse is missed out of the train of the cross pulses, a value of the output signal of the inter-pulse counter is two times the output signal value when all of the cross pulses are counted. Accordingly, an output signal of the divider 10 represents 2. Under this condition, an output signal of the -1 circuit 12 is 1, and the pulse generator 13 produces on shot of pulse. This pulse is applied through the OR gate 5 to the cross pulse counter 6. In this case, the pulse applied substitutes itself for the pulse missed from the cross pulse train. A sequence of the above operations may be charted as shown in FIG. 2. As seen, when the pulse subsequent to an (N+1) pulse of the cross pulse signal is missed, a count C of the inter-pulse counter 8 is 2 m, not the normal value "m". Since a count of the counter, when it counts an interval between the pulses N and N+1, is "m". The output signal of the divider 10 represents 2. The output signal of the -1 circuit 12 causes the pulse generator 13 to generate a pulse denoted as D.
Actually, however, the intervals each between the adjacent pulses of a train of track cross pulses are exactly equal to one another. Therefore, when one pulse is missed out of the cross pulse train, the pulse interval between the adjacent pulses is not always two times or larger than the previous pulse interval. When the pulse interval is smaller than the previous pulse interval, the conventional seeking control system does not produce a substitution pulse. Accordingly, the count by the cross pulse counter is incorrect. Where the optical head crossing the tracks is movable in a broad range of speeds, to obtain at a satisfactory resolution the pulse intervals of the cross pulses detected when the optical head is moved at a high speed, it is required that the frequency of the reference pulse must be set to be extremely high. If the pulse intervals obtained when the head is moved at a low speed is counted on the basis of the high frequency reference pulse, the count progresses at an excessively high frequency and therefore, some measure to prevent occurrence of overflow must be applied to the inter-pulse counter, count memory, divider and the like. That is, those components must be of the large-bit type. Use of such components leads to increase of cost to manufacture. Particularly, the divider per se is an expensive component. The increase of the cost becomes more marked.